I have been checking this code and trying to inject some code into my extension template admin interface.

Is it possible for example to inject some external css into admin template without modifying core template ?
That would be a huge advantage. It would allow to create custom admin interfaces within the current template. Is it possible ?

You can use template events to inject code, including INCLUDECSS and INCLUDEJS calls. Check this page and scroll to "Added ACP Template Events". Note that the list may be out of date, so check in the template file where you need an event and do a search for <!-- EVENT to see which events are available in that file. If there is not an event where you need one, you can request it in the [3.x] Event Requests forum (read the sticky). And, if you'd like, you're welcome to create a PR yourself to add the event so that it doesn't just get added to the backlog.

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